The present invention relates to methods of preventing acid mine drainage, treating acid mine drainage waters, removing metal and non-metal ions present in aqueous sources, and selectively recovering them from aqueous sources.
Acid mine drainage (AMD) is the phenomenon of acid production from mining tailings and frequently contains toxic metals. It results from the mining of precious metals (e.g., platinum, gold, and silver), radioactive metals (e.g., uranium and radium), and coal, and both working and defunct mines have AMD problems associated with them. AMD is a major environmental problem and has been a long-standing major concern to scientists, engineers, industry, and governments, both in this country and throughout the world.
AMD is produced by the oxidation of sulfide minerals (notably, pyrite) and ferric iron in the presence of a bacteriological catalyst, e.g., Thiobacillus ferrooxidans, producing ferrous sulphate and sulfuric acid by a complex series of chemical reactions.
The high acidity and the presence of toxic metals in AMD waters degrade soil, air, and water quality, and detrimentally impact vegetation and aquatic life. Consequently, mine waste waters, prior to being released into the environment, must be treated to meet government standards for the amount of metal and non-metal ions contained in the water. Some of these metals, such as uranium and selenium, which cause deleterious health effects, are extremely difficult to remove from mine waste waters.
Worldwide focus on AMD is increasing as countries become aware of the cost to industry and to the environment, especially since AMD is recognized as potentially impacting the environment more than subsoil acidity. One report has estimated that within the next ten years the Worldwide liability resulting from AMD will be between $10 to $20 billion. Meetings and conferences have been held Worldwide to discuss AMD, with no concrete solution to the problem.
In the past, efforts to treat mine waste waters have been ineffective or prohibitively expensive. Because treatment is ineffective at removing some metal and non-metal ions, mining throughput is sometimes restricted by government regulation. Because treatment is so expensive, no cleanup has occurred in many cases, especially for abandoned mines. Moreover, although some of the metal and non-metal ions abundant in mine waste waters are quite valuable, they cannot be economically extracted.
One attempted solution to AMD was simply to neutralize the mine waste waters with lime or limestone and then aerate. This method precipitates the iron, which is the metal most abundant in AMD waters, but other metals present are difficult to precipitate out and are not removed by this treatment. Also, remobilization of the contaminants can occur as well.
Other methods have treated mine waste waters with ionic surfactants to control the bacterial population and, thereby, inhibit the oxidation of ferrous iron to ferric iron. This strategy, however, has not been successful because many other factors, such as changing intraparticle diffusional resistances, are involved in controlling AMD, which this attempted solution does not take into account.
It has been hypothesized that phosphate could control AMD formation. However, this proposed solution is economically infeasible, since it has been discovered that high dosages are required. Further, the proposed phosphate system is pH sensitive, and when the pH decreases, the acid production cycle causing AMD is reinitiated.
A study has shown that sodium silicate at relatively high pHs can form ferrous and ferric precipitates. This approach, however, is uneconomical and results in remobilization of the contaminants in a way similar to that of using phosphate and limestone as described above.
Accordingly, there is an immediate need for a solution to the problem of AMD which overcomes the difficulties and failures described above.